1. Field of the Invention
This invention relates to a process for the recovery and/or isolation of organic carboxylic acids contained in aqueous solutions and more particularly to a process for their separation and/or isolation from each other or other materials as ester derivatives.
2. Description of the Prior Art
The art is well aware of processes for producing useful acids, such as adipic acid, by oxidizing napthenes, cycloaliphatic ketones or cycloaliphatic alcohols with nitric acid in the presence of metal compound oxidation catalysts. Such processes in general involve heating specific materials such as cyclohexane, cyclohexanol and/or cyclohexanone in nitric acid at about 40.degree. to 140.degree. C., generally utilizing nitric acid of about 20 to 90 percent strength, to produce a resulting oxidation mixture comprising the adipic acid together with small amounts of other dicarboxylic acids in admixture with the unused nitric acid and catalyst components. Generally, the art subsequently recovers a substantial amount of the desirable adipic acid by cooling the solution and filtering off the crystallized adipic acid. Such processes for producing the adipic acid are known, for example, in the prior art from U.S. Pat. Nos. 2,791,566; 2,840,607; 3,338,959; 2,971,010; 2,439,513; and 2,557,281.
Adipic acid, of course, is an important intermediate for the production of nylon by subsequent copolymerization with hexamethylenediamine to produce a polyamide capable of being spun into a fiber having a number of desirable characteristics. Many other uses for this material are known to the art.
While the art is well aware of how to recover the major amount of adipic acid, very little attention has been given to procedures for recovery of the remainder of the adipic acid as well as the other dicarboxylic acids contained in the resulting oxidate. My copending application Ser. No. 766,476 mentioned above provides a procedure for recovery of these additional acids as well as the other by-products contained in the mixture.
In the process for nitric acid oxidation of cyclohexanol and/or cyclohexanone, significant amounts of succinic acid and glutaric acid are formed as byproducts in admixture with the adipic acid. Various well known schemes of crystallization have been used heretofore in industry to attempt separation thereof and usually a substantial amount of the adipic acid can be removed by these procedures. Ultimately, however, a mother liquor is obtained from these operations which contains succinic acid, glutaric acid, and a small amount of adipic acid in such proportions that further concentration and crystallization steps will yield only mixtures of these dibasic acids. In addition, as a result of the removal of the adipic acid by crystallization and removal of the water and nitric acid by volatilization, the concentration of the metal catalyst in this final mother liquor becomes relatively high. Hence, loss of these materials, particularly the catalytic components, provides a distinct economic disadvantage as the relatively high proportion of valuable and reusable products is lost.
In my application above-identified, a novel procedure is provided by which the oxidate mixture may be treated with a water-miscible alcohol to esterify the acids present and the resulting esterified products may then be effectively extracted with a water-immiscible solvent such as benzene. The extracted esters may then be obtained as a mixture by removal of the solvent and distilled to separate the esters from each other. After separation, the esters may be easily hydrolyzed back to the individual acids.
In addition to this specific difficultly recoverable dibasic acid mixture however, there are known in the art various other aqueous mixtures of acids, which, because of their similar physical properties, are difficult to recover and isolate by methods known to the art. This is a particularly difficult problem when the acids are contained in dilute aqueous solutions. Such acid mixtures thus include the acid mixtures obtained in the byproduct stream in the air oxidation of cyclohexane to form cyclohexanol and cyclohexanone; the above-mentioned nitric acid oxidation of the cyclohexanol and cyclohexanone; the oxidation of fats and fatty acids which results in a complex mixture of acids with no acid being dominant (as described for example in U.S. Pat. No. 2,662,908); the oxidation of parafinnic hydrocarbons such as refined wax, semi-refined wax, petrolatum, lubricating oil, slack wax, scale wax, foots-oil, etc., in a two-stage process first using air, then nitric acid, which process produces mixtures of dibasic acids, and the like.
Heretofore it has been generally accepted that complex mixtures of this type could not be separated in a sufficiently economical manner to justify the expense involved. Accordingly, such acid mixtures have been used previously in their mixed form or treated as waste.
In addition, aqueous solutions are encountered where one acid will predominate but its physical and chemical characteristics make its isolation and purification difficult (e.g. lactic acid, levulinic acid, etc.). Moreover, it is often desirable to separate acids from water soluble impurities.
It has now been found, as will be described hereinafter, that the principle of the esterification/extraction technique disclosed in my above-identified copending application is applicable to the separation and isolation of many other classes of acids and mixtures thereof and this discovery forms the basis of this invention.